The D3300 requires lenses with focus motors in the lens, so several lenses will not auto focus on the D3300. Nikon AF-S lenses can focus, and the Sigma APO DG has such a motor, and it should auto-focus.
However, there are two routine ways that you can disable auto focus, when desired.
Many lenses have a switch on them to set Manual focus mode (lens has switch positions M or A, for Manual or Auto focus). Not certain about this lens, but probably has it. If present, make sure it is not on Manual focus. The B&H description of it says "full time Manual focus override", so it must have it. Check your instruction manual that comes with the lens.
Also the cameras also have a switch or menu to set Manual focus. Then no lens will auto focus (check to see if that is true now. If not true, then this is not the issue). For the D3300, setting this mode is described on page 74 of the Larger D3300 Reference Manual, available Free at http://downloadcenter.nikonimglib.com/en/products/21/D3300.html
This free Reference manual is like the User Manual, but 370 pages instead of 124 pages, much larger and more complete, more topics.
Either way will diable AutoFocus.